Various central nervous system disorders such as anxiety, depression etc., are believed to involve a disturbance of the neurotransmitter 5-hydroxytryptamine (5-HT) or serotonin. The actions of the neurotransmitter 5-hydroxytryptamine (5-HT) as a major modulatory neurotransmitter in the brain, are mediated through a number of receptor families termed as 5-HT1, 5-HT2, 5-HT3, 5-HT4, 5-HT5, 5-HT6 and 5-HT7. Based on a high level of 5-HT6 receptor mRNA in the brain, it has been stated that the 5-HT6 receptor may play a role in the pathology and treatment of central nervous system disorders. In particular, 5-HT6-selective ligands have been identified as potentially useful in the treatment of certain CNS disorders such as Parkinson's disease, Huntington's disease, anxiety, depression, manic depression, psychoses, epilepsy, obsessive compulsive disorders, migraine, Alzheimer's disease (enhancement of cognitive memory), sleep disorders, feeding disorders such as anorexia and bulimia, panic attacks, attention deficit hyperactivity disorder (ADHD), attention deficit disorder (ADD), withdrawal from drug abuse such as cocaine, ethanol, nicotine and benzodiazepines, schizophrenia, and also disorders associated with spinal trauma and/or head injury such as hydrocephalus. Such 5-HT6-selective ligands are also expected to be useful in the treatment of certain gastrointestinal (GI) disorders such as functional bowel disorder. (See for example, B. L. Roth et al., J. Pharmacol. Exp. Ther, 1994, 268, 1403-14120; D. R. Sibley et al., Mol. Pharmacol, 1993, 43, 320-327; A. J. Sleight et al., Neurotransmission, 1995, 11, 1-5; and A. J. Sleight et al., Serotonin ID Research Alert., 1997, 2 (3), 115-118, all of which are incorporated herein by reference).
Studies have found that a known selective 5-HT6 receptor antagonist may significantly increase glutamate and aspartate levels in the frontal cortex without elevating levels of noradrenaline, dopamine or 5-HT. This selective elevation of certain neurochemicals is noted during memory and cognition, strongly suggests a role for 5-HT6 ligands in cognition (Dawson, L. A.; Nguyen, H. Q.; Li, P., British Journal of Pharmacology, 2000, 130 (1), 23-26). Animal studies of memory and leaming with a known selective 5-HT6 receptor antagonist has some positive effects (Rogers, D. C.; Hatcher, P. D.; Hagan, J. J., Society of Neuroscience, Abstracts, 2000, 26, 680, all of which are incorporated herein by reference). A related potential therapeutic use for 5-HT6 ligands is the treatment of attention deficit disorders (ADD, also known as Attention Deficit Hyperactivity Disorder or ADHD) in children as well as adults. As 5-HT6 receptor antagonists appear to enhance the activity of the nigrostriatal dopamine pathway and ADHD has been linked to abnormalities in the caudate nuclei (Ernst, M.; Zametkin, A. J.; Matochik, J. H.; Jons, P. A.; Cohen, R. M., Journal of Neuroscience, 1998, 18 (15), 5901-5907), 5-HT6 receptor antagonists may attenuate attention deficit disorders. 5-HT6 receptor antagonists have also been identified as potentially useful compounds for treatment of obesity. See for example, Bentley et al., Br. J. Pharmac. 1999, Suppl 126; Bently et al., J. Psychopharmacol. 1997, Suppl A64, 255; Wooley et al., Neuropharmacology 2001, 41, 210-129; and WO02098878, all of which are incorporated herein by reference.